The invention relates to a transfer case for the drive train of a motor vehicle, which comprises a clutch unit by means of which it is possible to produce a connection between a first output shaft and a second output shaft.
In all-wheel drive vehicles, with one axle which is constantly driven and one axle whose drive can be connected and disconnected, a first requirement is for it to be possible to connect the second axle when driving, i.e. without interrupting the traction force or without having to stop to do this. A typical off-road situation occurs when, first of all, the wheels of the constantly driven axle start to slip, turning significantly more quickly than the undriven wheels. If the second driven axle is then to be connected without the load being interrupted (before the vehicle gets completely stuck), it is necessary to overcome a considerable difference in rotational speed and a high torque, which is impossible with an ordinary clutch.
A second requirement is economic. If the disengaged drive shaft of the undriven axle is also running, considerable losses and increased wear are generated. To avoid this, the drive train between the second driven axle and the transfer case is shut down. There is already a clutch at one end of the drive train, in the transfer case. If a clutch is also fitted at the other end of the drive train, i.e. in or on the second driven axle, this drive train can be shut down. However, if the second axle is to be driven while the vehicle is moving, not only it is necessary to overcome any difference in rotational speed during engagement in the transfer case and to transmit a moment, but also the entire drive train leading to the second axle has to be accelerated from a substantially stationary position.
Finally, a third requirement is that an all-wheel drive vehicle be compatible with electronically controlled brake systems, which for reasons of vehicle dynamics requires rapid and complete release of the connection between the two axle drive trains.
A transfer case of the generic type is known from U.S. Pat. No. 6,079,535. In the clutch unit described in that document, during engagement the first clutch part and the second clutch part, and with the latter also the entire drive train to the second driven axle, are accelerated by axle displacement of the pressure plate which is connected in a rotationally fixed manner to the first output shaft. Only when synchronous running is achieved is the claw clutch engaged, by further displacement of the pressure plate.
The fact that only the pressure plate is driven means that both clutch parts have to be accelerated via the frictional surfaces, during which process initially only that of the pressure plate on the closest disks is subjected to load, but consequently this load is particularly high. This causes noise, is only possible when driving very slowly and overall represents a load situation which is particularly unfavorable for the clutch and leads to rapid and also uneven wear.
When the drive train leading to the second axle is stationary, this solution is particularly disadvantageous, since the entire drive train has to be accelerated by means of initially only a few disks, which given a low surface pressure entails long acceleration times and therefore the risk of the clutch overheating, while if the pressure exerted is high (by means of a particularly strong second spring), the base moment of the multidisk clutch is high. The term base moment is to be understood as meaning the torque which occurs on initial contact of the disks. Consequently, the clutch action is very hard. Furthermore, this high base moment makes interaction with an electronic brake control unit difficult if not impossible, imposes high demands on the control system and during shifting, leads to stresses in the drive train.
Accordingly, it is an object of the invention to counteract these drawbacks and to improve a clutch of the generic type in such a way that it is possible to satisfy all three requirements mentioned above.
The invention relates to a transfer case for the drive train of a motor vehicle, which comprises a clutch unit by means of which it is possible to produce a connection between a first output shaft and a second output shaft, the clutch unit comprising:
a) a first clutch part, which is mounted rotatably on the first output shaft, can be displaced in the axial direction and has first clutch disks arranged in a rotationally fixed but translationally movable manner thereon,
b) a second clutch part, which is mounted so that it rotates coaxially with the first clutch part, is connected in a rotationally fixed manner to the second output shaft and has second clutch disks which are arranged in a rotationally fixed but translationally movable manner thereon,
c) a pressure plate which can be displaced axially toward the first clutch part, a first spring being provided between the pressure plate and the first clutch part,
d) a claw clutch, comprising a primary part, which is formed on the first output shaft or on a part which is connected in a rotationally fixed manner thereto, and a secondary part, which is formed on the first clutch part or on a part which is connected in a rotationally fixed manner thereto, which parts can be brought into engagement with one another counter to the force of a second spring.
According to the invention, the foregoing object is achieved by the fact that:
e) the force exerted by the first spring is greater than the force exerted by the second spring, and
f) the claw clutch is assigned a synchronizer clutch which comprises a first synchronizer part, which is connected in a rotationally fixed manner to the first output shaft, and a second synchronizer part, which is connected in a rotationally fixed manner to the first clutch part.
These two design measures have far-reaching consequences. The fact that the first spring is harder means that initially the first clutch part of the multidisk clutch is only displaced, without its disks coming into contact, (the stronger first spring prevents this from happening). However, the displacement counter to the force of the significantly weaker second spring brings the synchronizer mechanism into action, with the result that only the first clutch part is accelerated and is ultimately connected to the first output shaft via the claw clutch. Only then is the multidisk clutch closed gently and uniformly by further displacement of the pressure plate.
In this arrangement, the pressure plate is preferably guided in only an axially displaceable manner on the first clutch part. Since, because of the invention, a rotationally fixed connection to the first output shaft is not required, the pressure plate is at this point not itself transmitting any torque. Consequently, it can be of lighter and smaller design.
In a preferred embodiment, the first clutch part is inside the second clutch part and bears the inner disks, and the second clutch part is designed as a bell with adjoining second output shaft, the second output shaft being a hollow shaft mounted on the first output shaft. The lower moment of inertia of the first clutch part means that the time required for synchronization is shorter, and a design which is more favorable both with regard to the space required and with regard to the accessibility of the actuator is achieved. The same purpose is served if the claw clutch is arranged on that side of the first clutch part which is remote from the pressure plate.
To ensure that there is no drag torque produced in the multidisk clutch even under unfavorable conditions, and nevertheless to have short idle travels during actuation, the first spring is prestressed.
However, the invention also relates to a method for engaging a clutch unit, by means of which it is possible to produce the connection between a first output shaft and a second output shaft, the clutch unit comprising a multidisk clutch and a claw clutch with synchronizer clutch, the multidisk clutch comprising a first clutch part, which is mounted rotatably on the first output shaft, a second clutch part, which is connected in a rotationally fixed manner to the second output shaft, and the claw clutch comprising a primary part, which is formed on the first output shaft and a secondary part which is formed on the first clutch part or on a part which is connected in a rotationally fixed manner thereto.
The method according to the invention is intended to ensure that the three requirements mentioned in the introduction can be satisfied. It comprises the following steps:
a) firstly, the first output shaft accelerates the first clutch part by means of the synchronizer device,
b) when first output shaft and first clutch part are running synchronously, the claw clutch is engaged,
c) then, the multidisk clutch is used to couple the second output shaft to the first output shaft.
This procedure leads to the quickest possible connection of the second output shaft with minimal wear and the smallest possible requirement for space, as a result of the second drive train being connected, as it were, xe2x80x9cby installmentsxe2x80x9d, the individual installments also being optimally adapted to the prevailing conditions. By way of example, the lower moment of inertia of the first clutch part is accelerated using the smaller clutch, namely the synchronizer clutch.
A refinement of the method for engaging a clutch unit, with the first output shaft rotating and the second output shaft substantially stationary, consists in the fact that with the first output shaft rotating and the second output shaft substantially stationary, the second output shaft is only accelerated to the rotational speed of the first output shaft during coupling by means of the multidisk clutch. Consequently, the drive train of the second driven axle is brought up to speed within the shortest possible time.